1. Field of the Invention
The present invention relates to a high-frequency oscillation circuit, a high-frequency module, and a communication apparatus for transmitting/receiving high-frequency electromagnetic waves such as microwaves and millimeter waves.
2. Description of the Related Art
Hitherto, a high-frequency oscillation circuit as shown in FIG. 11 has been known. The high-frequency oscillation circuit is used for a communication apparatus or the like and includes a resonance circuit 101 and an active circuit 102 including a field-effect transistor (hereinafter, referred to as FET) for amplifying a signal (for example, Japanese Unexamined Patent Application Publication No. 7-86832). In the known high-frequency oscillation circuit, the resonance circuit 101 is formed by providing a substantially column-shaped dielectric resonator near a microstrip line on a surface of a board and the active circuit 102 is formed by using an amplifier circuit including the FET in which the drain terminal is grounded. Also, the microstrip line and the gate terminal of the FET are connected by wire bonding or bump. Accordingly, the dielectric resonator of the resonance circuit 101 outputs a signal of a predetermined frequency, and the active circuit 102 including the FET amplifies the signal so as to output the signal to a mixer or the like of a communication apparatus through an output terminal 103.
In order that the known high-frequency oscillation circuit may fulfill the oscillation conditions, the product of the absolute value of the reflection coefficient xcex931 of the resonance circuit 101 and that of the reflection coefficient xcex932 of the active circuit 102 must be 1 or more, as represented by the following expression 1.
|xcex931∥xcex932|1xe2x80x83xe2x80x83Expression 1
On the other hand, the absolute value of the reflection coefficient xcex931 of the resonance circuit 101 is generally less than 1 (|xcex931| less than 1). Therefore, the absolute value of the reflection coefficient xcex932 of the active circuit 102 must be more than 1 (|xcex932| greater than 1) in order to satisfy the expression 1. However, the impedance of the FET of the active circuit 102 greatly varies in accordance with the frequency of an input signal (for example, the impedance decreases as the frequency of the signal becomes higher). Thus, the absolute value of the reflection coefficient xcex932 may decrease in accordance with the frequency of the signal, and the oscillation conditions may not be fulfilled.
Also, since the microstrip line of the resonance circuit 101 and the FET of the active circuit 102 are connected by a wire bonding, the absolute value of the reflection coefficient xcex932 of the active circuit 102 may decrease due to the impedance of the wire. Accordingly, it may be difficult to achieve a stable oscillation.
The present invention has been made in view of the above described problems of the known art, and it is an object of the present invention to provide a high-frequency oscillation circuit in which oscillation conditions can be easily fulfilled and oscillation characteristics can be stabilized, and to provide a high-frequency module and a communication apparatus each comprising the high-frequency oscillation circuit.
In order to solve the above-described problems, a high-frequency oscillation circuit of the present invention comprises a resonance circuit; an active circuit including a field-effect transistor; and an impedance-conversion circuit for increasing the reflection coefficient of the active circuit. The impedance-conversion circuit is provided between the resonance circuit and the active circuit.
With this configuration, the impedance on the output terminal side of the resonance circuit can be converted so that the absolute value of the reflection coefficient of the active circuit increases, and then the converted impedance is connected to the active circuit. Accordingly, the oscillation conditions can be easily fulfilled and the oscillation characteristics can be stabilized.
The impedance-conversion circuit may be connected to the gate terminal of the field-effect transistor. In this case, the active circuit includes an amplifier circuit in which the drain terminal or the source terminal of the FET is grounded.
The impedance-conversion circuit may be formed by a pattern on a circuit board. With this arrangement, the impedance on the output terminal side of the resonance circuit can be easily converted by using the pattern of a microstrip line, etc. provided on the circuit board.
The impedance-conversion circuit may be formed by using an electrode for forming the field-effect transistor. With this arrangement, the impedance on the output terminal side of the resonance circuit can be easily converted by using the electrode of the field-effect transistor.
Alternatively, the impedance-conversion circuit may comprise a high-impedance line, one end thereof being connected to the resonance circuit; a capacitive stub connected to the high-impedance line; and a line for connecting the other end of the high-impedance line to the field-effect transistor.
With this arrangement, an inductive reactance can be set by using the high-impedance line and the connecting line, and a capacitive reactance can be set by using the capacitive stub. By combining the inductive reactance and the capacitive reactance, the impedance on the output terminal side of the resonance circuit can be converted to an impedance so that the absolute value of the reflection coefficient of the active circuit increases. Further, since the impedance-conversion circuit includes the connecting line, the variation in the oscillation characteristics caused by the connecting line can be absorbed by the impedance-conversion circuit, and thus the oscillation characteristics can be stabilized.
Alternatively, the impedance-conversion circuit may comprise a high-impedance line, one end thereof being connected to the resonance circuit; and a line for connecting the other end of the high-impedance line to the field-effect transistor.
With this arrangement, an inductive reactance can be set by using the high-impedance line and the connecting line. Accordingly, the impedance-conversion circuit can vary the phase of the impedance on the output terminal side of the resonance circuit. That is, the impedance can be adjusted to a low impedance which is approximate to the gate impedance of the FET. Further, since the impedance-conversion circuit includes the connecting line, the effect of the oscillation characteristics caused by the connecting line can be absorbed by the impedance-conversion circuit, and thus the oscillation characteristics can be stabilized.
In addition, a high-frequency module and a communication apparatus can be formed by using the high-frequency oscillation circuit according to the present invention.